PROJECT SUMMARY Of the estimated 1.6 million cycles of assisted reproductive technology (ART) performed globally in one year, nearly one third are categorized as frozen embryo transfer (FET). In the United States, over half of the ART cycles involve FET or freezing of all embryos. Given this widespread use of FET, it is of serious concern that multiple studies have demonstrated an association of FET with an increased risk for hypertensive disorders of pregnancy, including preeclampsia. It is critical that this risk be minimized because preeclampsia is associated with severe short- and long-term consequences for both mother and infant, including maternal mortality and fetal death. Furthermore, several recent meta-analyses emphasized that the evidence to guide clinicians about protocol choice for optimizing live birth rate with FET is poor. Our group has recently completed an NIH-sponsored Program Project Grant, with findings that strongly implicate one commonly performed FET protocol as an explanation for the increased risk of preeclampsia seen with FET. In a prospective cohort study, one protocol (the programmed FET) was associated with significantly higher rates of preeclampsia compared with an alternative protocol (the natural cycle FET). Furthermore, through detailed examination of maternal cardiovascular adaptation to pregnancy in two separate populations, perturbations of concern were seen with programmed FET, but not with natural cycle FET. Given these data and other findings in the literature, our central hypothesis is that pregnancy resulting from an FET in a natural cycle will have a lower rate of preeclampsia compared to an FET in a programmed cycle. To test this hypothesis, we will conduct a randomized, clinical trial at 6 sites. Our Primary Specific Aim is to determine the incidence of preeclampsia with the natural versus the programmed cycle. Our second hypothesis is that the live birth rate following transfer in a natural cycle is similar to that in a programmed FET, to be tested through a Second Specific Aim in which we compare the live birth rate with the natural versus the programmed cycle. All embryos will have been created prior to enrollment in the trial and no embryo will be subjected to increased risk in the course of this research. With successful completion of this protocol, we will have established the live birth rates for natural cycle FET compared to a programmed FET cycle, utilizing contemporary standardized protocols. If the natural cycle is confirmed to be associated with a lower risk of preeclampsia, this study will enhance the safety of ART for millions of women who are undergoing FET worldwide. This research proposal is highly responsive to the RFA and the overall mission of NICHD and NIH, as it will not only compare the live birth rates associated with two commonly used contemporary FET protocols, but it will perhaps even more importantly, compare the safety of these FET protocols with respect to maternal health.